Hydraulic modeling of a confined gas recirculation mixing system for a CSTR anaerobic digester
Hydraulic numerical rnadelling and experimental werk was used to develop a mixing system, that is able to satisfy the biologica! requirement of micro bacteria and create a homogeneaus salution inside the digester. A review was conducted on the different mixing methods used commercially for CSTR's as well as the influence of mixing intensity and intervals on anaerobic digesters. The gas recirculation method, eperating in a two-phase plug-flow regime mixing method was selected and a one-dimensional explicit, transient numerical model was developed that could predict the liquid mass displaced by a gas plug. An experimental study foliowed the numerical rnadelling of the digester mixing system to capture the effect of various geometrical constructions of the plug-flow generator in two empirica! equations to predict the mass and frequency of plug released. With the numerical model and experimental results a full scale mixing system was developed for a 1 600 m3 digester. The benefits of the proposed mixing system over the mechanica! draft tube methad include the following: no moving parts inside the digester, tedious downtime of clogged rotating equipment is eliminated and only requires 7 % more power per unit volume.